The revised International Classification of Epileptic Seizures categorizes absence seizures as generalized seizures, indicating bihemispheric initial involvement both clinically and electroencephalographically.2 Many children with absence seizures (formerly known as "petit mal") can be further categorized as having a characteristic epileptic syndrome.

Types of absence seizures

The terms typical and atypical absence seizures were used by the International Classification of Epileptic Seizures to describe and categorize the various absence types.

Typical absence seizures

The simple typical absence consists of the sudden onset of impaired consciousness, usually associated with a blank facial appearance without other motor or behavioral phenomena. This subtype is actually relatively rare. It comprised only 9% of 374 absence seizures video-recorded from 48 patients by Penry and associates.3

The complex typical absence is accompanied by other motor, behavioral, or autonomic phenomena:

Motor phenomena:

Clonic components may be quite subtle. They most frequently consist of eye blinking. Clonic activity may range from nystagmus to rapid jerking of the arms. Changes in tone often include a tonic postural contraction leading to flexion or hypertonic extension. Although a decrease in tone rarely causes a fall, it may lead to nodding of the head or the dropping of objects.

Behavioral phenomena:

Automatisms are the commonest clinical accompaniment, occurring in 44% of 476 typical absence seizures studied by simultaneous video-EEG radiotelemetry in 27 patients.4 Automatisms are semipurposeful behaviors of which the patient is unaware. The patient cannot recall them afterwards. There are two types:

• Perseverative, reflecting continuation of preictal activities.Complex activities such as dealing cards, playing patty-cake, or handling a toy are generally perseverative. Speech, if it occurs, also is usually perseverative. It is often slowed and dysarthric but may be totally normal and include both expressive and receptive aspects.
• Simple behaviors, such as rubbing the face or hands, licking the lips, chewing, grimacing, scratching, or fumbling with clothes, tend to be de novo automatisms.

Autonomic phenomena:

Autonomic phenomena associated with absence seizures may include:

• pupil dilatation
• pallor
• flushing
• sweating
• salivation
• piloerection
• urinary incontinence (not common)

Neither the autonomic changes nor automatisms allow one to distinguish absence from other seizure types.

Atypical absence seizures

Atypical absence seizures have traditionally been characterized4,5 as having:

• less abrupt onset or cessation than typical absence seizures
• more pronounced changes in tone
• longer duration
• age of onset before 5 years
• association with other generalized seizure types and mental retardation

Comparison of typical and atypical absence seizures

Holmes et al5 compared 426 typical and 500 atypical absence seizures in 54 children. The atypical absence seizure lasted significantly longer, on average, than the typical absence seizure. The most common initial clinical manifestation in either type was a change in facial expression or appearance of a blank stare. A pause or slowing of motor activity was also frequently noted as the initial finding in both types. The atypical absences were significantly more likely to have diminished postural tone or tonic or myoclonic activity as the initial clinical feature.

A blank stare or change in facial expression was the sole clinical finding in only 16% of typical and 28% of atypical absences. Automatisms, eye blinking, and lip smacking occurred more commonly in typical absences. These automatisms were usually perseverative, often consisting of playing with a toy or game. De novo automatisms were associated with longer spells and most commonly consisted of rubbing the face or hands or smiling.

A change in postural tone (either an increase or a decrease) was more commonly seen in atypical absences.

Both typical and atypical absences shared many characteristics:

• starting abruptly without an aura
• lasting from a few seconds to half a minute
• ending abruptly
• frequent eye blinking, lip smacking, decrease in tone, and automatisms

Although statistically significant differences can be identified, there is considerable overlap between the two seizure types. They most likely represent a clinical continuum. This overlap also pertains to the electroencephalogram (EEG) and proposed pathophysiology.

Typical absence seizures

The EEG signature of a typical absence seizure is the sudden onset of 3-Hz generalized symmetrical spike-and-wave or multiple spike-and-slow wave complexes. (See EEG.) The voltage of the discharges is often maximal in the frontocentral regions. The frequency tends to be faster (about 4 Hz) at the onset and slower (down to 2 Hz) toward the end of discharges if they persist longer than 10 seconds. The spike-and-wave discharge may be precipitated by hyperventilation or photic stimulation.

Ictal EEG of typical absence seizure of childhood absence epilepsy: Note the regular rhythm of the discharge, the constant spike and slow wave relation, and the abrupt onset. The opening phase is often variable and unreliable. The child remains unresponsive from the onset of the initial phase to the onset of the terminal phase of the discharge. However, she is able to understand the technologist during the terminal phase, when the ictal discharge is waning.

The interictal EEG background is often normal in typical absence seizures. Using the preceding ictal EEG criteria to classify absence seizures, Holmes et al.4 found that only 44% of 27 patients with typical absences had normal EEG backgrounds, however. Diffuse slowing was seen in 22% and paroxysmal spikes or sharp waves in 37%.

The discharges are more numerous during all sleep states except rapid eye movement (REM) sleep. During sleep the bursts are briefer and irregular, slowing to 1.5 to 2.5 Hz.

Hyperventilation, photic stimulation, and hypoglycemia will activate typical absence seizures. Hyperventilation is the most effective procedure.

Clinical effects generally can be seen when discharges last longer than 3 seconds. Detailed neuropsychologic investigations have demonstrated functional impairment from a spike-and-wave burst of any duration.6 Auditory reaction times were delayed 56% of the time when a stimulus was presented at the onset of the EEG paroxysm and were abnormal 80% of the time when the stimulus was delayed 0.5 second. Responsiveness may improve as the paroxysm continues.

Atypical absence seizures

In atypical absence seizures the ictal EEG is more heterogeneous, showing 1.5- to 2.5-Hz slow spike-and-wave or multiple spike-and-wave discharges, which may be irregular or asymmetric (See EEG.).

Slow spike-and-wave activity in a patient with atypical absence seizures.

The interictal EEG background is generally abnormal. In the study of Holmes et al,4 only 11% of 27 patients with atypical absences had a normal interictal EEG. Diffuse slowing and focal or multifocal spikes or sharp waves were seen in 85%.

Adapted from: Holmes GL. Classification of seizures and the epilepsies. In: Schachter SC, Schomer DL, eds. The comprehensive evaluation and treatment of epilepsy. San Diego, CA: Academic Press; 1997. p. 1-36.
With permission from Elsevier (www.elsevier.com)
Reviewed and revised January 2004 by Gregory L. Holmes, MD, Dartmouth Medical School

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